In typical low temperature fuel cells, of considerable interest in the automotive field, hydrogen (H2), or an organic material with high hydrogen content, supplied to the anode, is oxidized with the release of electrons, whilst at the cathode, oxygen (O2) is reduced. Platinum (Pt) is a highly active material acting as a catalyst in such fuel cells, and can be used at both the cathode and anode. At the cathode, platinum catalysts are commonly employed to assist in the oxygen reduction reaction (ORR).
A known format of a catalyst layer on a polymer electrolyte membrane (PEM) fuel cell cathode is shown in FIG. 1. The catalyst layer, shown here with a thickness of approximately 10 μm, contains platinum catalyst on a carbon support, intermixed with ionomer. Typical polymer electrolyte materials are perfluorosulfonic acid (PFSA) polymers, such as those commercially available under the trademarks Nafion®, Aquivion®, Flemion®.
The catalyst material, most typically Pt or Pt-alloy, notably in view of its stability in the very acidic conditions involved, promotes the oxygen reduction reaction by minimising the activation overpotential, facilitating the cathode reaction ½ O2+2 H++4e−→H2O, but can also be used at the anode to facilitate H2 oxidation, in the anode reaction: H2→H++2e−. Most preferably 2 to 5 nm diameter Pt or Pt alloy particles are used. The catalyst support provides an increased surface area to the Pt-based catalyst and should also provide electrical conductivity to transfer the electrons, a porous structure to enable gas diffusion and water transport, and stability in electrochemical conditions. The most common support materials are carbon-based, typically 30 to 100 nm average diameter activated carbon powder, such as commercially available Vulcan® or Ketjen® carbon materials. Usually the Pt catalyst is loaded at an amount of 10 to 60 weight percent with respect to the carbon support material.
A known method of preparing a Membrane Electrode Assembly by transferring a catalyst layer onto the polymer electrolyte membrane (so called “decal” process) is an “ink-based” process. It can be described as follows:                1) Preparation of the ink by mixing the supported catalyst (Pt/C), the ionomer (ion-conducting polymer) and an appropriate solvent;        2) Casting the ink on an intermediate hydrophobic substrate (such as PTFE-based membrane) that does not swell and that has a glass transition temperature higher than the polymer elelctrolyte membrane;        3) Drying at a temperature of less than 100° C. to eliminate the solvent, so that the catalyst layer is formed on the PTFE substrate;        4) “Decal” method: hot pressing of the PTFE substrate coated with a catalyst layer against the polymer electrolyte membrane (such as Nafion®); peeling off the PTFE substrate, so that the catalyst layer is transferred onto the polymer membrane forming a Membrane-Electrode-Assembly (MEA).        
There are disadvantages associated with this conventional catalyst layer structure, and in particular:                a) Because of the ink-based manufacturing process it is impossible to control the structure at micro/nano level ensuring the highest efficiency in Pt utilisation (by forming a maximum number of “three-phase boundary” sites). During the manufacturing process carbon particles may agglomerate, the ionomer may not be finely dispersed and may completely cover Pt particles making them inaccessible for catalytic reactions;        b) Carbon is generally not stable. During operation of the fuel cell carbon corrodes, and the Pt agglomerates show reduced surface area. To increase stability, the carbon could be graphitized, however in this case surface area would be lost and performance reduced;        c) The ionomer itself may promote Pt dissolution leading to performance loss.        
A structure for a fuel cell cathode catalyst layer not involving an “ink layer” is disclosed in U.S. Pat. No. 7,622,217. Here a nanostructured thin-film (NSTF) catalyst layer is disclosed wherein there is no carbon support, and no ionomer (ink layer). Organic crystalline whiskers act as a catalyst support, with a Pt-based catalyst loading, the catalyst comprising Pt in combination with other metals. An electrode layer of less than 1 μm thickness can be produced. However, the NSTF structure risks here being not mechanically robust, and the structure may not be retained during membrane electrode assembly (MEA) preparation and FC (fuel cell) operation. Through a loss of structure, disadvantages are to be expected, including both a loss in electrochemical surface area, with consequences on activation overpotential, as well as a loss of porosity with consequences for water-gas management and concentration overpotential.
In this context, it is desirable to develop new catalyst layers, which would be applicable in particular but not only in fuel cell applications, wherein one or more of the problems (a) to (c) listed above is minimized so that, as far as possible, a structure is provided having high catalytic activity and ability to support effective water and gas transfer, these properties being stable over the long-term use of the fuel cell.
Meanwhile, titania (TiO2) nanotubes are known as materials and can be prepared inter alia by anodization of titanium metal films. For example, CN 101560669 describes a method for preparing TiO2 nanotubes by anodization on a titanium metal substrate. A catalytic electrode is then prepared wherein a noble metal (such as Pt, Pd) is deposited on the nanotubes using pulse current combined with ultrasound.
WO 2010/080703 describes the preparation of nitrogen-doped titania nanotube arrays and their use in photocatalytic conversion of carbon dioxide into hydrocarbons in the presence of water vapour.
Titanium (di)oxide nanotubes (TNT) can also be prepared, apart from anodization techniques, by technologies involving treating titanium oxide with alkaline materials such as alkali metal hydroxides. Formation of a sol solution of titanium oxide in water, optionally with a lower alcohol cosolvent, can be carried out followed by treatment with a peroxide material such as hydrogen peroxide, followed by successive treatments with an alkali metal hydroxides and a cation source. Electrospinning can also be used to produce TiO2 nanotubes, and also TiO2 nanofibres, by using a core solution of a removable material such as mineral oil and a sheath solution of sheath material such as a titanium alkoxide (and other typical sol-gel precursor-type materials). The core solution of removable material and the sheath solution of sheath material are subjected to a high-voltage and forced through a spinneret. These various techniques for titania nanotube preparation are detailed in US 2010/0258446, which more specifically relates to catalytic materials having palladium dispersed on the nanotube surface, for use in photocatalytic conversion of carbon dioxide into reduced carbon compounds, such as methanol, in the presence of water vapour.